Cruise report from R/V Svea week 49-50, 2020

Type: Report
Author: Örjan Bäck, Lena Viktorsson


During the expedition, which is part of the Swedish national marine monitoring programme, the Skagerrak, the Kattegat, the Sound, the Baltic Proper and the Gulf of Bothnia was visited. The weather was overcast with moderate to fresh southerly winds in the Gulf of Bothnia and moderate to light winds in the Baltic Proper, Skagerrak and Kattegat.

The surface water (0–10 m) had cooled down since November. Nonetheless, the temperature was still above normal at all stations in the Baltic Sea, but within the normal range in the Skagerrak and Kattegat. The temperature was 7–8 °C in the Skagerrak and Kattegat, 8–9 °C in the Baltic Proper, about 7 °C in the Åland Sea, 5–6.5 °C in the Bothnian Sea and about 5 °C in the Gulf of Bothnia. The salinity of the surface water in the Baltic Proper was still above normal for the season and varied between 6.9–8.2 psu. In the Kattegat and Skagerrak, the salinity of the surface water was below normal to normal.

The concentrations of inorganic nutrients in the surface water had increased since November and were now close to winter maximums, which is normal for the autumn when phytoplankton activity decreases and the water mass mixes. Concentrations of silicate were higher than normal at most stations in the Baltic Sea and some of the stations on the west coast. Concentrations of phosphate were higher than normal in the Gulf of Bothnia and at a few stations in the Baltic Proper, the Skagerrak and the Kattegat. The concentrations of dissolved inorganic nitrogen (DIN) were below normal in the Bothnian Bay and above normal east of Gotland and in the Bornholm Basin, otherwise mostly normal. In the deep water in the Western Gotland Basin, the concentration of dissolved inorganic nitrogen was far above normal.

There was no oxygen deficiency in Skagerrak's bottom water. However, at the coastal station Släggö, the oxygen concentration had decreased to 2.54 ml/l. The oxygen concentration in the Kattegat's bottom water was slightly higher than in November, 5.0–5.5 ml/l. In the Sound, the oxygen concentration in the bottom water remained at the same level as in November, 3.8 ml/l.

As in November, there was no oxygen deficiency in the Arkona Basin, which is an improvement from October. At all other stations, on the other hand, there was acute oxygen deficiency or completely oxygen free (when hydrogen sulphide forms). In the Bornholm Basin, Hanö Bight and at the station BCS III-10, acute oxygen deficiency (<2 ml / l) was measured from 70 m to the bottom. At station BY15, there was no measurable oxygen in the water from a depth of 85 m and hydrogen sulphide was measured from a depth of 135 m. In the Western Gotland Basin, the depth at which hydrogen sulphide was first measured varied from 60 m at the station BY38 in the southern part of the basin to 125 m at the station BY31 in the northern part of the basin. At all other stations, hydrogen sulphide was measured from 80 m.

No oxygen deficiency was observed in the Gulf of Bothnia, which is normal. However, the oxygen levels in the deep water at some stations, primarily in the southern Bothnian Sea, were slightly lower than usual at this time of year.

The next regular expedition is planned 8-15 January with R/V Svea, then there is also winter mapping of nutrients in the Kattegat.